Last-stage buckets for steam turbines have for some time been the subject of substantial developmental work. It is highly desirable to optimize the performance of these later-stage buckets to reduce aerodynamic losses, particularly when it is recognized that the last stage of a steam turbine is the highest loaded stage and contributes on the order of about 10% to the overall output of the turbine. As will be appreciated, last-stage buckets are exposed to a wide range of flows, pressures, loads and strong dynamic forces. Optimally, the bucket profile should be designed to match aerodynamically the flow of the nozzle to provide the desirable operating characteristics over a large operating range. Factors which affect the final bucket profile include the active length of the bucket, its pitch diameter and its high operating speed in both supersonic and subsonic flows. Damping and bucket fatigue are factors which must be considered in the mechanical design of the bucket and its profile. The buckets must also be tuned to avoid coincidence between their natural frequencies and the flow stimuli. Additionally, the bucket profile must accommodate a smooth transition from subsonic flow adjacent the root to supersonic flow adjacent the blade tip. These mechanical and dynamic response properties of the buckets as well as others, such as thermodynamic properties or material selection all influence the optimum bucket profile. In brief, last-stage steam turbine buckets require a precisely defined bucket profile for optimal aerodynamic performance with minimum losses over a wide operating range.
Bucket designs in the past have included continuous coupling of the buckets at their outer tip employing under and over-covers as well as tiewires at intermediate stations along the buckets. These continuous couplings and tiewires are incorporated in the present bucket design to reduce bucket response to stimuli in the working fluid, which could cause uncontrolled vibration of the buckets, for example, at their natural frequencies. Vibration, of course, is to be minimized or eliminated to avoid fatigue and eventual structural failure and these continuous couplings and tiewires, of course, affect the aerodynamic properties of the buckets. It is important also to seal the tips of the buckets to minimize aerodynamic loss from flow passing around the bucket tips. The appropriate bucket profile is also important to provide converging-diverging flow passages between adjacent buckets and untwisting of the buckets from an ambient over-twisted configuration to a desired profile configuration at rated operation condition to achieve maximum aerodynamic efficiency.